Aqueous dispersions of polyurethanes have been known, i. a., from U.S. Pat. No. 5,334,651. These polyurethanes are based on reaction products of polyfunctional isocyanates, polyols having a number average molar mass of at least 400 g/mol, and compounds having at least two groups which are reactive towards isocyanate groups, and at least one group capable of anion formation, which are reacted to form an isocyanate-functional prepolymer, and which prepolymer is reacted in a second step with at least one of low molar mass polyols, and compounds which have one active hydrogen atom, or more than one active hydrogen atom wherein these said hydrogen atom differ in reactivity towards isocyanate groups. These polyurethanes are hydroxy-functional at least due to the reaction of the second step, and aqueous dispersions thereof can be cured with isocyanate crosslinking agents to give tough and resilient films.
Coating films prepared from these polyurethanes can still be improved particularly with respect to their solvent resistance and resistance to soiling and graffiti.
It has been found in the investigations leading to this invention that a particular choice of starting materials, or educts, together with a certain combination of the specific amount of substance of hydroxyl groups, of the specific amount of substance of urea groups, and of the degree of branching in the molecules constituting the hydroxy functional polyurethanes provides the desired combination of hardness, as measured by the König method, anti-graffiti properties, and solvent resistance, of coating films prepared from the said polyurethanes in a two-pack composition, including adapted crosslinkers therefor.
The invention therefore relates to an aqueously dispersible polyurethane having    (aa) a specific amount of substance of silicon n(Si)/m(PU) of from 0.05 mol/kg to 1 mol/kg, and    (cc) a specific amount of substance of urea groups, n(U)/m(PU),>N—CO—N<  (formula I)     of from 0.8 mol/kg to 2 mol/kg,wherein in each case the specific amount of substance is calculated as the ratio of the amount of substance n of the entity under consideration (atoms such as Si and C, groups of atoms such as>N—CO—N< and —OH), to the mass m(PU) of the polyurethane.
In a preferred embodiment, the polyurethane may additionally also have one or both of the following properties:    (bb) a degree of branching measured as the sum of the specific amounts of substance of tertiary aliphatic carbon atoms C3 and of quaternary aliphatic carbon atoms C4, (n(C3)+n(C4))/m(PU), of from 0.01 mol/kg to 0.5 mol/kg, and    (dd) a specific amount of substance of hydroxyl groups, n(—OH)/m(PU), of from 1 mol/kg to 4 mol/kg.
The invention is also directed to a process for the preparation of the aqueously dispersible polyurethane PU according to the invention, comprising the steps of    (a) preparing a mixture A of at least two hydroxy-functional polymers A, having a number average molar mass of at least 400 g/mol, and, on average, more than one hydroxyl group per molecule, where i is an integer number from 1 to at least two, which means that there are at least two hydroxy functional polymers A1 and A2 and wherein at least one of the hydroxy-functional polymers A, is a silicon-containing polyol A1,    (b) mixing the mixture A of hydroxy-functional polymers with either or both of (b1) a hydroxy-functional or amino-functional acid B1 which has at least one, and preferably two, functional groups selected from the group consisting of hydroxyl groups, primary amino groups, and secondary amino groups, and at least one acid group, and (b2) a polyether B2 which has oligo-oxyethylene groups obeying the formula II—O—(—CH2—CH2—O—)n—CH2—CH2—O—,  (II)     wherein n is from 1 to 100, and at least one, preferably two hydroxyl groups, or with either or both of (b3) a hydroxy-functional or amino-functional basic compound B3 which has at least one, and preferably two, hydroxyl or primary or secondary amino groups and at least one basic group which is preferably a tertiary amino group, and (b4) a polyether B2 which has oligo-oxyethylene groups obeying the formula II—O—(—CH2—CH2—O—)n—CH2—CH2—O—,  (II)     wherein n is from 1 to 100, and at least one, preferably two hydroxyl groups,    (c) reacting the mixture prepared in step (b) with at least one polyfunctional isocyanate C having at least two isocyanate groups per molecule, wherein the amount of isocyanate C is chosen such that there is a ratio of isocyanate groups in component C to hydroxyl groups present in the mixture prepared in step (b) of from 2:1 to 1.1:1, until at least 90% of the hydroxyl groups of the mixture prepared in step (b) are consumed by reaction with the isocyanate component C,    (d) adding to the reaction product of step (c) at least one of a hydroxyamine D having at least one primary or secondary amino group, and at least one hydroxyl group, a polyhydric alcohol E having at least two hydroxyl groups per molecule, and a polyfunctional amine F having at least two amino groups, each of the amino groups being selected from primary and secondary amino groups,    (e) dispersing the reaction product of step (d) in water,wherein the amounts of the reactants A, A1, B1, B2, B3, C, D, E, and F are chosen such that the resultant polyurethane PU fulfills the conditions (aa) and (cc), as detailed supra, and optionally, either or both of the conditions (bb) and (dd).
In the context of the present invention, the mixture A of hydroxy functional polymers having, on average, more than one hydroxyl group per molecule, and preferably, at least two hydroxyl groups per molecule, preferably comprises a mass fraction of at least 1%, and up to 60%, of a silicon-containing hydroxy functional polymer A1 of formula III(HO—R5—)k-qR7qT′-(SiR1R2—O)n—(SiR3R4)p-TR8r(—R6—OH)m-r  (III)wherein    R1 is an alkyl having from one to ten carbon atoms, and may be linear or branched, or an aryl having from six to twenty carbon atoms which may also be substituted by one or more alkyl or alkoxy groups having from one to ten carbon atoms, and which latter alkyl or alkoxy groups may be linear or branched,    R2 is selected from the same group as R1, but independently thereof,    R3 is selected from the same group as R1, but independently thereof,    R4 is selected from the same group as R1, but independently thereof,    R5 is an alkylene group which may be linear or branched, having from one to ten carbon atoms, which may also be substituted by one or more alkyl or alkoxy groups having from one to ten carbon atoms, and which latter alkyl or alkoxy groups may be linear or branched,    R6 is selected from the same group as R5, but independently thereof,    R7 is selected from the group consisting of a hydrogen atom, an alkyl group which may be linear or branched, having from one to twenty carbon atoms, and an aryl, alkaryl, and aralkyl group each having independently from each other from six to thirty carbon atoms,    R8 is selected from the group consisting of a hydrogen atom, an alkyl group which may be linear or branched, having from one to twenty carbon atoms, and an aryl, alkaryl, and aralkyl group each having independently from each other from six to thirty carbon atoms,    T is a direct bond or an atom selected from the group consisting of O, N, or an aliphatic or aromatic group with m+1 binding sites which are not on the same carbon atom in the case of T being an aliphatic or aromatic group, and m may be from 1 to 4, and is 1 for O, and 2 for N, r being an integer of from 0 to m−1, and T not being O if p is 0,    T′ is a direct bond or an atom selected from the group consisting of O, N, or an aliphatic or aromatic group with k+1 binding sites which are not on the same carbon atom in the case of T′ being an aliphatic or aromatic group, and k may be from 1 to 4, and is 1 for O, and 2 for N, q being an integer of from 0 to k    n is from 2 to 100, and    p is 0 or 1.
According to these conditions, the silicon-containing hydroxy-functional polymer A1 has at least one hydroxyl group in its molecule.